Characterization of mechanochemically synthesized imidazolates of Ag+1, Zn+2, Cd+2, and Hg+2: Solid state reactivity of nd10 cations

Article abstract of DOI:10.1016/j.jpcs.2006.02.006

Silver, zinc, cadmium, and mercury imidazolates have been synthesized mechanochemically by milling imidazole and the metal oxides in an agate mortar. The reaction products were characterized by FTIR and XRPD techniques. The results obtained for the mechanochemical imidazolates have been compared with those obtained by precipitation reported in the literature. The mechanochemical Ag imidazolate has the same orthorhombic crystal structure as the precipitated one. The mechanochemical Zn imidazolate has a tetragonal structure with similar crystal parameters to those of Zn(Imz)₂H₂O, but no water molecules are present in the structure. This new anhydrous form is a polymorph of the one obtained by precipitation. The mechanochemical Cd imidazolate has a monoclinic structure which is the polymorph of the precipited orthorombic form. The mechanochemical Hg imidazolate presents a hexagonal structure which is a polymorph of the orthorombic structure obtained by precipitation. The influence of the nd¹⁰ electronic configuration of the cations on the mechanochemical reaction is discussed.

Full text of DOI:10.1016/j.jpcs.2006.02.006

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Journal of Physics and Chemistry of Solids 67 (2006) 1612–1617
www.elsevier.com/locate/jpcs
Characterization of mechanochemically synthesized imidazolates of
Ag+1, Zn+2, Cd+2, and Hg+2: Solid state reactivity of nd10 cations
Ã
J.F. Ferna
´
ndez-Bertra
J. Rodriguezb, A. Panequea, J.C. Llopizb
na, M.P. Hernandezb, , E. Reguerab,c, H. Yee-Madeirad,
´ ´
aCenter of Pharmaceutical Chemistry, 21 Avenue and 200 Street, Playa, P.O. Box 16042, Havana, Cuba
bInstituto de Ciencia y Tecnologia de Materiales, Zapata y G, P. O. Box 10400, Universidad de La Habana, Cuba
cCenter of Applied Science and Advanced Technologies of IPN. Mexico, D.F., Mexico
´ ´
Escuela Superior de Fısica y Matematicas-IPN, UP ‘‘ALM’’, Colonia Lindavista 07738, Mexico
d
´
Received 16 January 2006; received in revised form 4 February 2006; accepted 10 February 2006
Abstract
Silver, zinc, cadmium, and mercury imidazolates have been synthesized mechanochemically by milling imidazole and the metal oxides
in an agate mortar. The reaction products were characterized by FTIR and XRPD techniques. The results obtained for the
mechanochemical imidazolates have been compared with those obtained by precipitation reported in the literature. The
mechanochemical Ag imidazolate has the same orthorhombic crystal structure as the precipitated one. The mechanochemical Zn
imidazolate has a tetragonal structure with similar crystal parameters to those of Zn(Imz)2H2O, but no water molecules are present in the
structure. This new anhydrous form is a polymorph of the one obtained by precipitation. The mechanochemical Cd imidazolate has a
monoclinic structure which is the polymorph of the precipited orthorombic form. The mechanochemical Hg imidazolate presents a
hexagonal structure which is a polymorph of the orthorombic structure obtained by precipitation. The influence of the nd10 electronic
configuration of the cations on the mechanochemical reaction is discussed.
r 2006 Elsevier Ltd. All rights reserved.
Keywords: A. Polymer; B. Chemical synthesis; C. Infrared spectroscopy; C. X-ray diffraction; D. Crystal structure
1. Introduction
nanometric materials with interesting physico-chemical
properties and promising applications in the fields of
mechanical and thermal stability, absorptive and cataly-
tical properties, electronic, magnetic, optical, anticorrosive
and biocidal activity, protein staining, etc. [2–4 and
references therein] The imidazolate anion (see Fig. 1) acts
as a rigid angular exobidentate ligand forming open
zeolitic-type structures [4].
Imidazole is a small nitrogen heterocycle molecule of
paramount biological importance. Its five membered ring
includes an acidic pyrrolic NH group and a basic pyridinic
N [1] (see Fig. 1). As a part of hystidine, imidazole is the
binding site for cations in metalloproteins. In neutral
solutions, imidazole forms complexes with transition metal
salts retaining the anion in the structure while in basic
solutions it forms imidazolates with the loss of the pyrrolic
proton and the incorporation of the cation with formation
of polymeric species [1].
Imidazolates are normally obtained by mixing
a
water-soluble salt of the metal with a basic imidazole
solution. The product precipitates in microcrystalline form
and is easily filtered, washed and dried. The imidazolate
polymers are prone to appear in different polymorphic
forms [1,4–7]. The synthetic procedure, nature of the
solvent and of the metal salt used can have an effect on
the crystalline structure of the products. For example,
Co(II) imidazolates crystallize in seven different poly-
morphic forms [4,5], while Cu(II) imidazolates have five
Imidazolates do not only have importance as model
compounds for metalloproteins but as metal–organic
Ã
Corresponding author. Fax: +53 7 2736471.
E-mail addresses: fernandezbertran@yahoo.com
(J.F. Fernandez-Bertran), mayrap@fisica.uh.cu (M.P. Hernandez).
´ ´ ´
0022-3697/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2006.02.006

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the elimination of unreacted imidazole. The samples still
contain the excess of unreacted oxides. For this reason, the
XRPD analysis had to consider two phases, the oxide and
the reaction product.
The FTIR spectra were run in an ATI Mattson ‘‘Genesis
Series’’ FTIR spectrometer. The samples were run in KBr
disks and also in Nujol mull on NaCl windows to detect
possible reaction of the imidazolate cations with the Brꢀ
anion of the KBr matrix during the milling and pressing of
the disk. Only AgImz reacted with KBr forming KImz
[10,15,19].
Fig. 1. The equilibrium between imidazole (ImzH), imidazolium cation
(ImzH2)+ and imidazolate anion Imzꢀ.
The XRPD patterns were run in a Siemens D5000
˚
diffractometer using Cu(Ka) radiation (l ¼ 1:54183 A). All
polymorphs [6]. Polymorphism is an important property of
imidazolates.
data were collected, at room temperature, in the 5–601 (2y)
ranges, with a step scan of 0.021 (D2y).
Mechanochemistry is a rapidly expanding field which
has the advantages of simple grinding of the reagents and
absence of solvents [8–11]. Our group has worked in this
field, on acid–base reactions [12–15], synthesis of novel
compounds [16–18], oxidation–reduction reactions [19],
adducts and complex formations [20,21].
We have obtained the imidazolates of copper, silver, zinc
and mercury by simple hand milling imidazole with oxides
in an agate mortar. Cd, Ga and In oxides require stronger
milling in mechanic grinders [15]. The common feature of
the reactive oxides is to have a cation with nd10 outer
electronic configuration.
Very basic oxides such as Al, Mg, Ca and Ba did not
react. This means that the first step of the mechanochem-
ical reaction is not removal of the acidic pyrrolic H by
oxides but the complexation of the pyridinic N by the
cation and the subsequent liberation of the pyrrolic proton.
Oxides of Fe+2, Mg+2, Cu+2,Co+2 or Ni+2 which are
strong complexing cations do not react mechanochemi-
cally, so the nd10 configuration is decisive for the reaction
[15]. The structure of the mechanical imidazolates has not
been reported due to the low crystallinity of the materials.
This is due to the effect of milling on the crystals, which
introduces defects, stresses and breaking of polymeric
chains, leading to partial amorphization of the sample.
In this paper we present the mechanochemical synthesis
and characterization of Ag(I), Zn(II), Cd(II) and Hg(II)
imidazolates, comparing the FTIR spectra and XRPD
patterns with those obtained by precipitation reported in
the literature [22–25].
The standard peak-search methods implemented in the
Win-Plotr Program [26] were used for location of the
diffraction maxima. The data were indexed using the trial-
and-error algorithm implemented in the TREOR program
[27]. A posteriori, the proposed crystal symmetry and
lattice parameters obtained from TREOR were confirmed
using the Le Bail method [28].
The application of the Le Bail method is based on the
following considerations: the background level has been
modeled by a third-order polynomial fitting; the profile
shape has been described by a pseudo-Voigt function; the
angular dependence of the full-width at half-maximum
(FWHM) of the observed peaks for each phase was
modeled by a tanðyÞ dependent function. The agreement
factors (Rp, Rwp and Re) were defined according to
Ref. [26].
3. Results and discussion
3.1. Infrared results
FTIR spectra are an easy and fast way to monitor the
mechanochemical reactions of imidazole with oxides [15].
The FTIR spectra of solid imidazole is characterized by a
strong
broad
absorption
band
covering
the
3200–2500 cmꢀ1 region, due to the NH stretching vibra-
tions of the pyrrolic N–H groups strongly associated to the
pyridinic N of neighboring molecules by strong hydrogen
bonds (HB). The broad band is modulated by gaps due to
Evans windows caused by Fermi resonance of the
fundamental NH stretch with forbidden harmonics and
combination bands [29,30]. On milling the imidazole with
the oxide the band disappears if there is reaction and
remains intact if not [15].
2. Experimental section
The products used in the synthesis of imidazolates were
reagent-grade chemicals from Merck, BDH and Sigma
companies. The mechanochemical synthesis using imida-
zole and the proper oxide in stoichiometric amounts was
carried out by hand milling of small samples (o1 g) in an
agate mortar for 20 min. CdO reacted very slowly, the
reaction being incomplete after 30 min. The mechanochem-
ical samples were studied by FTIR and XRPD after these
samples were washed and dried at 50 1C for 24 h allowing
In Fig. 2, we present several spectra which illustrate the
formation of imidazolate by the shifting of the sharp
intense band dCH of imidazole at 1055 cmꢀ1 to the region
of 1075–1095 cmꢀ1 of the imidazolate [31]. These pair of
bands shed light on the progress of the reaction as well as
on the nature of the product [15].
Table 1 summarizes the values of the dCH band for the
different mechanochemical imidazolates obtained.

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precipitated imidazolates reported in the literature. They
are considered in the order Ag(I), Zn(II), Cd(II) and Hg(II)
imidazolates.
3.2.1. Silver imidazolate
The Ag(Imz) polymer has been reported by Masciocchi
[22], as a white stable precipitate obtained by mixing
AgNO3 solution with imidazole–NH3 solution. The poly-
mer is orthorhombic, See Table 2.
The mechanochemical product obtained by milling silver
oxide and imidazole was washed to remove excess of
unreacted imidazole. The dried product shows two phases,
silver oxide and AgImz polymer. The mechanochemical
imidazolate has the same crystalline structure as the
precipitated one. This is rather surprising and is indicative
of the stability of this crystalline form (see Table 2 and
Fig. 3). The supramolecular structure is determined by
nd10–nd10 interactions between adjacent chains with
˚
Ag–Ag distances of 3.16 A [22].
3.2.2. Zinc Imidazolate
Lehnert [23] has reported a tetragonal structure for
Zn(Imz)2 obtained by mixing Zn acetylacetonate with a
mixture of imidazole and tetramethyl imidazole. Xiaochun
[24] has obtained a similar imidazolate by mixing
Zn(ClO4)2 solution with imidazole. The crystal parameters
of the two products are equal as can be seen in Table 2.
Tian [4] has reported a hydrate Zn(Imz)2 H2O of
tetragonal structure, but with different space groups and
structures and slightly larger value of c and lower values of
a and b parameters.
We have obtained Zn(Imz)2 mechanochemicaly with
similar structure to the I41 Zn(Imz)2H2O phase, but the IR
spectra indicate that no water molecules are present in the
structure [15]. In consequence, the mechanochemical phase
is a polymorph of the I41cd phase (precipitated) [23,24]. See
Table 2 and Fig. 4.
Fig. 2. FTIR spectra of imidazole and Zn(II), Cd(II) and Hg(II)
imidazolates (mechanochemical). Region of dCH band.
Table 1
Vibrations in the finger print region of mechanochemical imidazolates in
KBr-pressed disks (cmꢀ1
)
Assignment
Zn(II)Imz2
Cd(II)Imz2
Hg(II)Imz2
dCH
1317
1238
1169
1088
951
1327
1234
1161
1086
937
1311
1232
1161
1088
935
Ring vibration
Ring breathing
dCH
Ring bending
Ring bending
gCH
829
758
829
756
823
762
t
667
667
660
3.2.3. Cadmiun imidazolate
Masciocchi [25] obtained cadmium imidazolate by
precipitation, mixing Cd(ClO4)2 solution with aqueous
imidazole. They report an orthorhombic phase (see
Table 2).
The XRPD pattern of our mechanochemical product is
complex and of poor crystallinity. This probably has to do
with the low speed and poor yield of the mechanochemical
process with CdO. The peaks-search methods located 23
diffraction maxima of which three intense peaks corre-
spond to CdO. TREOR gave for the monoclinic system the
Ag(I), Zn(II), Cd(II) and Hg(II) imidazolates show
identical FTIR spectra for the mechanochemical and
precipitated species except for the presence of oxide bands
in the former. This is due to the fact that IR bands in the
finger-print region consist of atomic motions of the rigid
imidazolate ring. The finger-print region is not useful for
detecting polymorphism, only the region below 400 cmꢀ1
which contains the N-M vibrations is useful for this
purpose [4]. The IR spectra also confirm the absence of
hydrated species.
˚
˚
approximate cell dimensions, a ¼ 15:41 A, b ¼ 12:96 A, c ¼
˚
10:76 A and b ¼ 134:001, [Mð20Þ ¼ 38 [32], Fð20Þ ¼
3.2. XRPD results
70:ð0:004149; 69Þ [33]]. The monoclinic structure was
confirmed using the Le Bail Method (see Table 2 and
Fig. 5). The monoclinic phase has a cell volume of
The XRPD characterization of the mechanochemical
imidazolates is not an easy task due to the poor crystal-
linity of the product and the complex nature of the reaction
mixture. Our results are compared with those of crystalline
3
˚
1536:6 A , nearly equal to the reported volume of
1556:2 A for the orthorhombic phase [25]. See Table 2
and Fig. 5.
3
˚

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Table 2
Values of the lattice parameters of the precipitated and mechanochemical imidazolates
3
V (A )
˚
a (A)
˚
b (A)
˚
c (A)
˚
M(i)Imzi
AgImz
Ref.
Syntheses
Space group
ang (deg.)
[22]
This work
Precipitated
Mechanochemical
P212121
P212121
5.5759
5.577
6.7452
6.737
22.174
22.13
a; b; g ¼ 90
a; b; g ¼ 90
834.0
831.2
ZnImz2
[23]
[24]
Precipitated
Precipitated
I41cd
I41cd
I41
23.481
23.572
22.874
22.895
23.481
23.572
22.874
22.895
12.461
12.467
12.941
12.988
a; b; g ¼ 90
a; b; g ¼ 90
a; b; g ¼ 90
a; b; g ¼ 90
6869.3
6926.0
6771.0
6808.0
[4]
This work
Precipitated monohydrate
Mechanochemical
I41
Cd Imz2
Hg Imz2
[25]
This work
Precipitated
Mechanochemical
Pbca
P2/m
14.605
15.382
10.739
12.920
9.923
10.749
a; b; g ¼ 90
a; g ¼ 90
1556.2
1536.6
b ¼ 134:0
[25]
This work
Precipitated
Mechanochemical
Pbca
P6/mmm
14.590
12.092
10.808
12.092
9.820
12.383
a; b; g ¼ 90
a; b ¼ 90
g ¼ 120
1548.4
1530.4
Fig. 3. The XRPD pattern of mechanochemical AgImz is shown
superposed to the calculated by Le Bail method. Two phases are present,
Ag imidazolate and AgO (*). The results of agreement factors were
Rp ¼ 10:0, Rwp ¼ 13:0 and Re ¼ 10:3.
Fig. 4. The XRPD pattern of mechanochemical ZnImz2 is shown
superposed to the calculated by Le Bail method. Two phases are present,
Zn imidazolate and ZnO (*). The results of agreement factors were
Rp ¼ 6:91, Rwp ¼ 10:01 and Re ¼ 7:69.
3.2.4. Mercury imidazolate
3.3. The role played by the nd10 outer electronic
configuration of the active cation in the mechanochemical
process
Masciocchi [25] obtained mercury imidazolate by mixing
solutions of Hg(CH3COO)2 and ammoniacal solution of
imidazole. They reported an orthorombic phase. See
Table 2.
Our mechanochemical reaction mixture presented 48
diffraction maxima, eight belonging to the Pnma orthor-
ombic phase of HgO and 40 to the mechanochemical
mercury imidazolate. TREOR led to a hexagonal system
for the polymer phase with approximate cell dimensions,
The cations of all the oxides which are active mechan-
ochemical reagents with imidazole have nd10 outer electro-
nic configurations. The d electrons are not as penetrating as
the s and p ones, producing a very polarizable electronic
cloud. Also, the nd10 electrons are very close energetically
to the ðn þ 1Þs0 and ðn þ 1Þp0 empty orbitals favoring
hybridization [34].
In the case of imidazolate–Ag+1 interactions the
hybridization of nd with ðn þ 1Þs and ðn þ 1Þp orbitals of
Ag+1 leads to a dz2 spz hybrid with strong directional
orientation toward the sp2 lone electron pair of the
pyridinic N. The strong overlapping of these orbitals
12:10 ꢁ 12:10 ꢁ 12:40
[Mð40Þ ¼ 38
[32],
Fð40Þ ¼
84:ð0:006106; 78Þ [33]]. The Le Bail method confirmed the
hexagonal structure of the imidazolate (see Table 2
and Fig. 6). The cell volumes of the mechanochemical
and precipitated phases are very similar, 1530.4 and
3
˚
1548.4 A .

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4. Conclusions
The crystal symmetry and lattice parameters of Ag(I),
Zn(II), Cd(II) and Hg(II) imidazolates obtained by milling
the oxides with imidazole in an agate mortar are reported
for the first time and compared with those of crystalline
imidazolates obtained by precipitaton. The FTIR spectra
of the mechanochemical and precipitated imidazolates are
similar.
The mechanochemical AgImz has the same crystal
structure as the precipitated imidazolate. The mechan-
ochemical Zn(Imz)2 presents the same structure as
Zn(Imz)2H2O, but devoid of H2O. Cd(II) and Hg(II)
imidazolates obtained mechanochemically are different
crystalline phases of the precipitated imidazolates reported
in the literature.
Fig. 5. The XRPD pattern of mechanochemical CdImz2 is shown
superposed to the calculated by Le Bail method. Two phases are present,
Cd imidazolate and CdO (*). The results of agreement factors were
Rp ¼ 7:59, Rwp ¼ 10:2 and Re ¼ 8:23.
Acknowledgments
Our group is grateful to Dr. Norberto Masciocchi for the
constant help received during our work. Also to Drs.
J. Elguero and H. Xiaochun for valuable information. To
G. Quintana for running the FTIR spectra and to
J. A. Gallegos for obtaining the XRPD patterns.
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